FIG. 1 represents a representative "slice" of color space in the visible spectrum, showing the additive and subtractive primary colors. (As is known in the art of color science, the slice illustrated in FIG. 1 is a section of a three-dimensional color space, with a white-to-black neutral axis emerging from the center of the diagram out of the page.) Around the perimeter of the section of color space are shown locations representing a full saturation of the subtractive primary colors yellow (Y), magenta (M), and cyan (C). These subtractive colors, as is well known, are used in the printing of images, because combinations of these colors can theoretically simulate all other colors in the visible spectrum.
Located between the various pairs of subtractive primary colors in the color space are what are here called the "hi-fi colors," blue (B), red (R), and green (G). As can be seen in the Figure, each hi-fi color can theoretically be simulated by combining, such as on a printed surface, colorants (such as toner or ink) of the two adjacent primary colors, so that magenta plus cyan together on a printed sheet would yield blue, while cyan and yellow would produce green, and so forth. However, particularly in the field of xerography, where images are typically formed with dry powders which are fused onto a paper sheet, this theoretical mixing of primary colors to yield other colors may result in a limited printer gamut. The inevitable chemical shortcomings of typical toner compositions will often cause combinations of primary color toner to yield a sub-optimal rendering of the desired combination color.
Within the shape shown in the Figure, wherein colors of any kind will be more vivid (i.e., have higher chroma) as one approaches the perimeter, the shaded area bounded by solid curved lines represents a typical practical gamut of colors obtainable with the printing apparatus. If it is attempted to print one of the primary colors, such as yellow, yellow toner is simply applied to the sheet, unalloyed with any other color; in such a case, pure yellow toner will yield the theoretical maximum chrominance of the desired color. This optimal use of uncombined primary color is represented by the fact that the solid curved line within the shape meets the perimeter of the shape only at the point of pure color, when yellow toner is not combined with any magenta or cyan toner.
However, if it is desired, for example, to print a green area, there must be a supplied onto the paper a visually-effective combination (either dots next to dots, or one toner layer on another) of yellow with cyan. As long as one primary color dominates, almost optimal chrominance can be achieved, as is shown by the fact that the curved solid line is reasonably close to the shape when yellow or cyan dominates. When colors toward a pure green are desired, which would require close to a half-and-half split of the two types of toner, the lack of chrominance becomes noticeable, as shown in the Figure by the fact that, near the area marked G, the solid curved line is quite far from the corner of the shape which represents a perfect green. In practical terms, the fact that the solid curved line is far from the perimeter of the shape results in a distinct dull or grayish appearance when the combination color is attempted. A similar lack of chrominance will appear when other hi-fi hues, such as red or blue, are attempted to be printed with close-to-equal proportions of subtractive colors.
It has been proposed, particularly in the art of xerographic printing, to overcome the problem of obtaining the hi-fi hues by providing printing apparatus which lays down not only the CMY primary colors, but also one or more apparatus (such as development units) which lay down one or more types of toner to print hi-fi hues, such as RGB. Instead of trying to obtain, for example, pure blue by mixing magenta and cyan toners, such a system would simply lay down a dedicated blue toner. With the addition of a black (K) development unit such as for printing of text, such "hi-fi" color printing systems would typically include five or more development units.